1. Field of the Invention
The present invention relates to a wireless communication system, and particularly to an antenna apparatus in a MIMO (Multiple Input Multiple Output) system, which conducts wireless communications using a plurality of antennas for transmitting and receiving.
2. Description of the Related Art
A received signal r in a wireless communication is given as a result of integrating the product of a directivity G(θ, φ) of an antenna and a characteristic H(θ, φ) of a propagation path, as shown in the equation below.r=∫0π∫−ππG(θ,φ)H(θ,φ)dφdθ  (1)
In the above equation, θ and φ respectively represent an azimuth angle and an elevation angle in the receiving side. Generally, because H(θ, φ) is a variable of natural phenomenon which a human can not control, and expresses an incoming wave distribution which is spatially polarized. Accordingly, it is desired that human controllable G(θ, φ) is spatially uniform (nondirectional antenna) for improving a received power, however, it is not practicable to construct such an antenna.
As a result, a case occurs that a directivity of an antenna is null in the direction from which radio waves are coming. In this case, the received power is greatly reduced while received electric field density is high.
The Patent Document 1 below relates to an antenna apparatus comprising a plurality of antennas having different directivities for solving the above drawback. In this antenna apparatus, an output obtained from the directivity synthesis of four monopole antennas and an output of a patch antenna are compared, and the output with the better communication quality is selected.
Patent Document 1
Japanese Patent Application Publication No. 2004-266367
In recent years, as a technique of improving frequency utilization efficiency, a MIMO system has gathered interests in the field of a wireless communication, in which system communications are conducted by a transmitting side and a receiving side both of which have a plurality of antennas respectively. In the transmitting side in the MIMO system, a spatial multiplexing is conducted by simultaneously transmitting different information from the plurality of transmission antennas at the same frequency. In the receiving side, signals are received by the plurality of reception antennas, and information is extracted by demultiplexing the respective signals.
However, the above conventional antenna apparatus has the problems below.
In the MIMO communication, it is necessary that the respective antennas are arranged such that fading correlations between the antennas are sufficiently low in the receiving side because the spatially multiplexed signals are to be demultiplexed. When the plurality of antennas are arranged simply in an array, the volume of the antenna apparatus becomes greater. However, when the intervals of antennas are reduced for a smaller size of the antenna apparatus, the fading correlations become higher, and the characteristic deteriorates.
In the antenna apparatus of the Patent Document 1, four monopole antennas are recognized as one antenna having the first directivity, and a patch antenna is recognized as the other antenna having the second directivity in order to selectively use only one of these two antennas. Accordingly, the spatial multiplexing can not be conducted by the plurality of antennas, therefore, the antenna apparatus of the Patent Document 1 is not suitable for the MIMO communication.